The invention relates to a sheet item feeder comprising:
at least three surfaces consisting of:                at least one circulatable feeding surface of which at least a portion faces in a first direction transverse to the support plane for frictionally engaging a sheet item from the stack and movable in a feeding direction transverse to said first direction in the course of the circulation for exerting traction to that sheet item; and        at least one separation surface of which at least a portion faces in a second direction opposite to the first direction for frictionally engaging the sheet item or an entrained next sheet item from the stack;        
wherein, in a lateral direction transverse to the feeding direction and to the first and second directions, at least the at least one feeding surface is located between two of the separation surfaces or the at least one separating surface is located between two of the feeding surfaces.
For separating sheets from a stack by exerting traction to the sheets to be separated, several principles of operation are known. In the field of preparation of items to be mailed, in which mostly printed sheets with varying properties have to be processed, two important separation principles that are used are friction separation (also referred to as automatic separation) and gap separation.
In friction separation, a separating surface is typically pressed elastically against a feeding surface. The suspension of the friction coefficient of the separating surface is such that it is entrained with the feeding surface if no sheet material or only a single layer of sheet material is present between the feeding surface and the separating surface. If two sheets are present between the feeding surface and the separating surface, the traction between the separating surface and the nearest sheet is larger than the friction between the two sheets so the nearest sheet, which is in contact with the separating surface, is prevented from being entrained by the moving sheet on the side of the feeding surface.
In gap separation a gap is provided between the feeding surface and the separating surface. The width of the gap is such that only a single sheet at a time is entrained by the friction surface through the gap between the friction surface and the separating surface. If one or more additional sheets are fed to the gap the additional sheet or sheets engage the separating surface which prevents the additional sheet or sheets on the side of the separating surface from being entrained through the gap until the previous single sheet passing through the gap has cleared the gap. The gap may be adjusted so that multi-layered items, such as folded sheets, sheets that are bound to each other or envelopes can be passed through the gap, one at a time only, from a stack of items that are all of generally the same thickness.
Accordingly, in the present context, the term “sheet item” is used to also encompass generally flat, sheetlike items, such as a folded sheet, a booklet, a folder, a cards, an envelope, a carrier carrying a plastic card or a flat data carrier, such as a CD or DVD in a pouch. Where the items are multi-layered, such as envelopes, the layers need to be sufficiently fixed relative to each other to not shift to the extent of being damaged or causing a jam when subjected to oppositely oriented friction forces for feeding and separating.
While ease of use is an important advantage of friction separation, friction separation is relatively unreliable when separating sheet material that is difficult to separate, such as coated (“glossy”) sheets that tend to cling to each other or multi-layered sheet items of which the layers can become dislodged relative to each other under influence of opposed traction forced exerted to the layers of a sheet item. On the other hand, while gap separation is more reliable when it comes to separating some types of sheet material that are difficult to separate and multi-layered sheet items, its performance depends heavily on an adequate adjustment of the size of the gap and the need of providing a very fine adjustment for adjusting the size of the gap complicates the design of such separating mechanisms.
In U.S. Pat. No. 2,635,874 an apparatus of the initially identified type is disclosed. In this sheet item feeder, the feeding and separating surfaces, constituted by circumferential surfaces of feeding and separating rollers, are not arranged opposite of each other, but staggered in lateral direction transverse to the feeding direction. This causes sheets passing between the rollers to be bent to some extent into a pattern that is wavy in lateral direction. Because the sheets do not pass between a gap between the feeding rollers and the separating rollers, such separating mechanisms are less sensitive to a precise adjustment of the positions of the feeding and separating surfaces. However, the performance of such systems nevertheless depends on adequately adjusting the positions of the feeding and separating surfaces to the stiffness of the sheet material, both laterally and in directions transverse to the plane in which the sheets are transported.
In the apparatus disclosed in U.S. Pat. No. 2,635,874, the sensitivity to adjustment of the relative positions of the feeding surfaces and the separating surfaces is reduced by providing that a spring loaded mechanism urges separating rollers against stationary surfaces opposite of the separating rollers so that the counter force resisting sheets from being entrained is exerted between the separating rollers and a counter surface. A disadvantage of such a system is that it is relatively complicated and that a sheet to be fed also encounters resistance from the stationary counter surface against which it is pressed by the pressure exerted by the separating rollers.